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e3c682ab52
in thumb mode, r7 is the ABI frame pointer register, and unless frame pointer is disabled, gcc insists on treating it as a fixed register, refusing to spill it to satisfy constraints. unfortunately, r7 is also used in the syscall ABI for passing the syscall number. up til now we just treated this as a requirement to disable frame pointer when generating code as thumb, but it turns out gcc forcibly enables frame pointer, and the fixed register constraint that goes with it, for functions which contain VLAs. this produces an unacceptable arch-specific constraint that (non-arm-specific) source files making syscalls cannot use VLAs. as a workaround, avoid r7 register constraints when producing thumb code and instead save/restore r7 in a temp register as part of the asm block. at some point we may want/need to support armv6-m/thumb1, so the asm has been tweaked to be thumb1-compatible while also near-optimal for thumb2: it allows the temp and/or syscall number to be in high registers (necessary since r0-r5 may all be used for syscalll args) and in thumb2 mode allows the syscall number to be an 8-bit immediate. |
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